A cellular communication system has become the mainstream of mobile telecommunication systems such as mobile phone systems. With the cellular communication system a plurality of areas (cells) in each of which a base station can perform transmission and receiving are combined to perform communication in a wide area. When a mobile station moves, a base station is switched to continue communication.
At present third generation mobile telecommunication services based on a CDMA (Code Division Multiple Access) system have been begun. A next generation mobile telecommunication system which enables higher-speed communication is widely discussed.
In 3GPP (3rd Generation Partnership Project), on the other hand, LTE (Long Term Evolution) and a high-speed radio service referred to as LTE-advanced which is a further developed version of LTE are discussed. With LTE-advanced the introduction of a relay station is discussed as a technique for enhancing throughput or improving characteristics in a dead spot.
To make it impossible for a mobile station to recognize the presence of a relay station is possible. In 3GPP, however, to make a relay station operate in the same way as an ordinary radio base station operates is mainly discussed. In this case, an upper base station to the relay station functions from the relay station as a mere connection point like a router.
A technique for reducing the number of times a mobile station performs transmission in a radio communication system in which a relay station is located by reducing the amount of signaling is proposed as a prior art (Japanese Laid-open Patent Publication No. 2009-81513 (Paragraphs [0035]-[0047], FIGS. 1 and 2)).
With LTE or LTE-advanced base stations may communicate with each other for hand-over or interference control. A communication interface between base stations is prescribed as an X2 interface.
FIG. 17 illustrates interfaces among base stations. A radio network 5a includes base stations eNB0 through eNB4. With the X2 interface in LTE base stations are connected by wire. In the case of FIG. 17, the base station eNB0 is connected to the base stations eNB1 through eNB4 via wired transmission lines X2-1 through X2-4 respectively.
For the sake of simplicity only interfaces between the base stations eNB0 and eNB1, between the base stations eNB0 and eNB2, between the base stations eNB0 and eNB3, and between the base stations eNB0 and eNB4 are illustrated. In reality, however, each base station is connected to another base station. As a result, mesh-like connections are made.
When the base station eNB0 communicates with another base station, the base station eNB0 uses wire in accordance with the X2 interface. For example, when the base station eNB0 communicates with the base station eNB1, the base station eNB0 uses the wired transmission line X2-1. When the base station eNB0 communicates with the base station eNB2, the base station eNB0 uses the wired transmission line X2-2.
Usually communication between base stations based on the X2 interface is performed in this way by wire. However, if the above relay station which operates in the same way as a base station operates is included, the relay station and an upper base station are connected by radio. As a result, a radio connection is made in a part of an interval based on the X2 interface.
FIG. 18 illustrates interfaces among base stations in a radio network including a relay station. A radio network 5b includes base stations eNB0 through eNB4, a relay station RN, and a mobile station UE.
The relay station RN operates in the same way as an ordinary base station operates. In addition, there is an upper base station (base station eNB0), which is also referred to as a donor, to the relay station RN. The relay station RN is connected to the upper base station eNB0 via a radio transmission line X2-5. The mobile station UE is under the control of the relay station RN.
The relay station RN communicates with the base stations eNB1 through eNB4 via the upper base station eNB0. Accordingly, not only wired communication but also radio communication is performed. For example, when the relay station RN communicates with the base station eNB1, the radio transmission line X2-5 and a wired transmission line X2-1 are used.
FIG. 19 is a sequence diagram of hand-over. Hand-over is indicated as an example of communication between the relay station RN and another base station. It is assumed that the mobile station UE under the control of the relay station RN in the radio network 5b performs hand-over and that candidate hand-over destinations are the base stations eNB1 and eNB2.
(S101) When the mobile station UE performs hand-over, the mobile station UE measures receiving levels of radio waves transmitted from surrounding base stations as a result of the movement, adds measurement results to signaling referred to as a measurement report, and transmits the signaling to the relay station RN.
(S102) When the relay station RN receives the receiving level measurement results, the relay station RN recognizes that the receiving levels of radio waves transmitted from the base stations eNB1 and eNB2 are high and that the base stations eNB1 and eNB2 are candidate hand-over destinations. It is assumed that the relay station RN first transmits an HO request (hand-over request signaling) to the base station eNB1. The HO request is transmitted to the base station eNB1 via the upper base station eNB0.
(S103) When the base station eNB1 receives the HO request, the base station eNB1 determines from its congestion state and the like whether or not hand-over to the base station eNB1 is possible. If hand-over to the base station eNB1 is possible, then the base station eNB1 returns an HO OK (hand-over enable signaling). If hand-over to the base station eNB1 is impossible, then the base station eNB1 returns an HO fail (hand-over disable signaling). In this case, the base station eNB1 determines that hand-over to the base station eNB1 is impossible, and returns an HO fail to the relay station RN. The HO fail is transmitted to the relay station RN via the upper base station eNB0.
(S104) When the relay station RN receives the HO fail from the base station eNB1, the relay station RN transmits an HO request to the base station eNB2 which is the other candidate hand-over destination. The HO request is transmitted to the base station eNB2 via the upper base station eNB0.
(S105) When the base station eNB2 receives the HO request, the base station eNB2 determines whether or not hand-over to the base station eNB2 is possible. If hand-over to the base station eNB2 is possible, then the base station eNB2 returns an HO OK to the relay station RN. The HO OK is transmitted to the relay station RN via the upper base station eNB0.
(S106) The relay station RN determines from the contents of the HO OK that the base station eNB2 is a hand-over destination, and gives the mobile station UE notice to that effect by an HO command (hand-over command). The mobile station UE then recognizes from the contents of the HO command that the base station eNB2 is a hand-over destination base station, and performs hand-over to the base station eNB2.
When the relay station RN communicates with another base station, radio communication is performed via the radio transmission line X2-5 between the relay station RN and the upper base station eNB0. However, if interference from another station, interference with another station, and the like are taken into consideration, it is desirable that the amount of radio signaling on the radio transmission line X2-5 should be reduced. That is to say, it is desirable that the amount of radio signaling exchanged between the relay station RN and the upper base station eNB0 or the number of times radio signaling is exchanged between the relay station RN and the upper base station eNB0 should be small.
In the above hand-over sequence, however, hand-over is performed without taking the amount of radio signaling into consideration. Accordingly, radio signaling is frequently exchanged between the relay station RN and the upper base station eNB0 before a hand-over destination base station is determined.
As a result, the amount of radio signaling on the radio transmission line X2-5 increases, the level of interference from another station or interference with another station rises, and communication quality deteriorates. In addition, with radio communication there is a long delay in a process such as establishment of a radio line. Accordingly, if radio communication is performed frequently, processing delay increases.
In the above description hand-over is taken as an example. However, the same problem may occur in communication other than hand-over between the relay station RN and another base station.